Point-to-multipoint (1:n) communications systems are now prevalent in the field of wireless communication, which allow one device to communicate with two or more devices simultaneously. The existing standards for such 1:n wireless communication include, for example, IEEE 802.16d defining fixed wireless access systems and IEEE 802.16e defining mobile wireless access systems (see, for example, the following Literatures 1 and 2). In this section, the term “radio base station” is used to refer to a device that can communicate simultaneously with a plurality of devices, and the term “mobile stations” to refer to the devices that communicate with the radio base station.
Literature 1: The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” IEEE 802.16-2004.
Literature 2: The Institute of Electrical and Electronics Engineers (IEEE), “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems,” IEEE 802.16e-2005.
Many of the 1:n wireless communication systems are designed to operate under the primary control of radio base stations. For example, the allocation of radio resources used in communication between a radio base station and mobile stations is centrally managed by the radio base station. Here the radio base station may use an adaptive modulation and coding (AMC) technique to enhance the efficiency of wireless communication. With AMC, the radio base station dynamically determines which modulation and coding scheme (MCS) to use to communicate with a mobile station, depending on the current quality of radio links between the radio base station and the mobile station.
A modulation and coding scheme specifies, for example, a modulation method, a coding method, and a coding rate. By combining different options for those elements, a number of candidates for the modulation and coding scheme are made available. Each such candidate has a different transmission rate, or in other words, each candidate transports a different amount of data per unit radio resource. The radio base station is supposed to select an appropriate modulation and coding scheme for individual mobile stations, from among those having different transmission rates. For example, it may be appropriate to select a modulation and coding scheme with a high transmission rate for mobile stations having a high radio link quality. It may also be appropriate to select a modulation and coding scheme with a low transmission rate for mobile stations having a low radio link quality, because such mobile stations would otherwise encounter more frequent data errors and consequent instability of communication.
Mobile communications systems may use multicarrier modulation techniques, in which case the modulation method and coding rate are selected based on power levels of subcarriers. Specifically, the radio base station measures the power level of each received subcarrier signal and selects a set of subcarriers capable of achieving a certain grade of transmission rate. The radio base station then determines which modulation method and coding rate to use, according to the receive power level of the selected subcarriers (see, for example, Japanese Laid-open Patent Publication No. 2003-304214).
While the modulation and coding schemes selected in the above-described adaptive modulation and coding may be advantageous to the radio base station itself, it does not always mean that the same schemes are also advantageous to mobile stations.
More specifically, the radio base station will be able to transmit the same amount data with a fewer radio resources and thus increase the number of simultaneous communication sessions, by applying a modulation and coding scheme with a high transmission rate to mobile stations with a high link quality. This means that, from the viewpoint of radio base stations, it is more advantageous to select a modulation and coding scheme with as high a transmission rate as possible.
The mobile stations, on the other hand, have to raise their output power to transmit data to the radio base station by using a modulation and coding scheme with a high transmission rate. This leads to an increased power consumption in the mobile stations. The use of a modulation and coding scheme with a high transmission rate also affects the reception of data from the radio base station. That is, the probability of successful reception is reduced as the transmission rate increases. This means that, from the viewpoint of mobile stations, it is more advantageous to select a modulation and coding scheme with a lower transmission rate, even under the condition of good radio link quality.